Spacecraft, such as satellites, shuttles, etc., typically employ one or more types of deployable devices or systems or structures, such as solar panels, antennas, etc. These deployable devices are secured to the spacecraft using some type of deployment system (e.g., a hinge or other mechanism) designed to maintain the deployable device in a stowed position during the launch of the spacecraft, and to facilitate the deployment of the device to a deployed or expanded position when needed once the mission of spacecraft is underway.
Prior art deployment systems used to secure the deployable device to the spacecraft differ somewhat in their features and configurations, but most comprise several common elements, such as an actuating device, a principal axis, a fixed support base, a deployable arm, a position sensor, a stop or stopper, and some type of latching or locking device.
In many prior art deployment systems that utilize a hinge mechanism, springs are the means most commonly used to generate the torque and energy needed to cause the deployable device to go from a stowed position from a deployed position, but motor-driven systems are also utilized. Within these spring loaded hinge mechanisms, stoppers and latches are typically used to halt and lock the deployable motion, respectively. In essence, the stopper functions as a barrier to halt the motion of the deployable mechanism, while the latch or lock functions to prevent maintain the deployable device in its deployed position, as well as to prevent back driving or counter rotation due to rebounding. An end-of-travel latch may be used to keep the deployable devices from oscillating during spacecraft maneuvers and interfering with the attitude control systems of the spacecraft. For instance, spacecraft maneuvers can cause the deployable device to swing which may induce periodic dynamic loads. Utilizing a latch will ensure that the spacecraft and the associated deployable devices maintain a cohesive overall structural stiffness and rigidity.
Latches and stoppers themselves may differ in their design and configuration, but most are intended to fulfill the same general function, namely to maintain the deployable device in its stowed position. Providing a positive latch at the end of travel, in addition to a stop, is the most common configuration. However, these types of latches require a sufficient amount of energy or force to actuate, which energy may be provided by the spring near or at the end-of-travel of the deployable device. In other words, the spring energy must be sufficient to overcome the resistance present within the latch. The problem is further compounded when one considers that spring energy typically decreases as the spring is allowed to approach a state of equilibrium, which in the case of a deployment system for a deployable device is as the deployable device nears the deployed position.
Other prior art deployment systems eliminate the use of a latch at the end of travel and rely solely on the force exerted by the spring to hold the deployable device against the stop. Although the elimination of a latch reduces overall system complexity, there is a significant drawback in that in order to provide a high strength restraint at the end of travel, the spring energy would need to be sufficiently high, which would undesirably cause excessive end-of-travel loads during the deployment phase. To absorb the additional loads, some type of dampener or other limiting element may be needed, which would ultimately increase the complexity of the system and create other problems.
The types of latches that are the most common in the industry include, magnetic latches, over-center latches, snap-action latches, motor driven latches, and sprocket mechanisms. Magnetic latches are most often associated with furniture cabinets as to provide a simple form of locking. The latch consists of a magnet located on the door of the cabinet and a mating magnet held on the frame of the door. Such latches are simple in design and easy to assemble. Magnetic latches may be used in space applications, but within such applications they present several significant drawbacks. First, the magnetic field generated by the magnets may interfere with on-board electronics. Second, this same magnetic field may also negatively affect the attitude control system of the spacecraft (e.g., it may add perturbation to the attitude control system) by creating magnetic torque between the magnet and the earth's natural magnetic field. Third, the magnets may lose some of their strength as a result of the conditions they are subjected to, such as temperature variations or high shocks.
Over-center latches utilize rigid bar linkage to provide a counter retaining force. This type of latch can be found on the retractable landing gear systems of common aircraft. Over-center latches utilize a complex configuration of moving parts, and require extensive manufacturing process control, assembly of the many component parts, and low tolerances.
Snap-action latches are designed to passively snap a moving body into a cavity, groove or hole or other form of mating receptacle. A good example of a snap-action latch can be found on any door that will automatically latch when pushed against the doorframe. Snap-action latches may passively be engaged due to their geometry but will need an external load, such as a door handle, to be disengaged. For a deployable device that uses a hinge mechanism actuated by a torsion spring, a significant amount of energy may be required to actuate the snap-action latch at end of travel. This requires that the spring must be configured to comprise enough energy to counteract the resistance from the latch. This added energy may be sufficient to induce a shock within the system if some type of shock absorbing element is not present.
Motor-driven latches require a motor to drive, in a linear or circular motion, a pin, finger or wedge that will close to retain or lock the deployable device. The main advantage of using a motor resides in the capability of the deployable device to be fully controlled by the operator to achieve any number of deployed positions. The same can be said for any form of active system that may be controlled to achieve multiple positions. An electrical motor, magnetic pin puller, a hydraulic actuator or any other form of active mechanism may drive the latch. The drawback for such a system is the need for an external controlling unit to drive the deployable device. In addition, motor-driven latches may require gear train or mechanical coupling configurations that significantly increase the complexity of the system and the potential for malfunction. Indeed, these types of latches require more parts, have high operational risks, are expensive, and utilize complex electromechanical interactions.
A sprocket mechanism is another alternative system capable of preventing back driving or counter rotation of the deployable device. This simple design may easily be implemented around a shaft and is widely found in many applications, such as tie-downs, blinds, tooling, bikes and other systems or structures. Sprocket mechanisms provide free pivoting or rotation in one direction, while preventing rotation in an opposing direction. While sprocket mechanisms are easy to implement, they have one major drawback, which is backlash or back drive produced before reaching the locked or latched position. Backlash can be detrimental to a deployable device in that its stiffness and pointing accuracy are adversely affected. This is particularly important in those circumstances in which the actuation spring is fractured and unable to maintain an actuation force on the deployable device, which force would otherwise reduce the degree of backlash.
Additional latching mechanisms, such as wedge latches, cam latches, and leaf latches are also provided for in some space applications. Each of these utilize mechanical resistance or friction to operate, which resistance can lead to undesirable results.